These holes are caused by bubbles that are produced by bacteria called Propionibacter during the late stage of cheese production. The larger the eyes, the better flavor of the cheese. Swiss cheese with no eyes are known as "blind."

Hi All.In the case of "Emmenthaler" the Propionibacter inside is the reason for them BIG holes. In other Products (like Alp-Cheese, Appenzeller, Tilsiter...) the holes are smaller and fewer than in the "Emmenthaler". All kinds of different "Good-Bacterias" (Milchsäurebakterien) can help to create "taste". Simply said: they "eat" the Milksugar and digest it to gas which creates the holes. YES! You're right. The fart is the hole. And we simply say "Loch" to the hole;)

@Guido-To be sure, what Americans call "Swiss Cheese" refers to a *specific* sort of cheese (Emmentaler), not all cheese from Switzerland. The term "Emmentaler" is pretty much unknown in the US, though -- the guy at the sandwhich shop will never know that term. For sure, Switzerland produces other cheese besides this, but here "Swiss cheese" ALWAYS means "Emmentaler" to 99.9% of the population.

Similarly, there are many varieties of cheese produced in the U.S., but "American Cheese" always means nasty processed factory cheese "stuff". Cheddar, Colby, &c produced within the US are commonly labelled with State of origin (New York, Wisconsin, Vermont are typical).